Mr. Muhammad and his followers had been killed by the C.I.A., the first time it had deployed a Predator drone in Pakistan to carry out a “targeted killing.” The target was not a top operative of Al Qaeda, but a Pakistani ally of the Taliban who led a tribal rebellion and was marked by Pakistan as an enemy of the state. In a secret deal, the C.I.A. had agreed to kill him in exchange for access to airspace it had long sought so it could use drones to hunt down its own enemies.

Rachel Maddow features the Justice Department paper, "Lawfulness of a Lethal Operation Directed Against a U.S. Citizen"

The United States can lawfully kill a U.S. citizen overseas if it determines the target is a “senior, operational leader” of al-Qaeda or an associated group and poses an imminent threat to the United States, according to a Justice Department document.

The document defines “imminent threat” expansively, saying it does not have to be based on intelligence about a specific attack since such actions are being “continually” planned by al-Qaeda. “In this context,” it says, “imminence must incorporate considerations of the relevant window of opportunity” as well as possible collateral damage to civilians.

It says that such determinations can be made by an “informed, high-level official of the U.S. government.”